The tetrapyrrole biosynthesis pathway in E. coil is branched. Glutamyl-tRNA participates in protein synthesis as well as in heme biosynthesis. E. coli appears to have only one glutamyl-tRNA synthetase and only one kind of tRNA glu. Another branch point occurs at the uroporphyrinogen III step and it leads to siroheme or heme. These considerations suggest that the control of this pathway requires interesting strategies which are worth investigating.